Biochar Addition Changes the Aggregation of Clay Mineral and Natural Soil Nanoparticles via Different Mechanisms

Wide application of biochar and subsequent release of biochar nanoparticles (NPs) significantly impact the stability of natural clay minerals and soil NPs, which are crucial for soil quality and play a vital role in determining the fate of nutrients and contaminants in the environment. Soil is a naturally occurring complex system composed of multiple components. Existing research on soil particle aggregation has predominantly focused on homo-aggregation. However, the addition of biochar to soil inevitably induces hetero-aggregation with soil components. In this study, the effects of biochar NPs on the aggregation of representative clay minerals (hematite and illite) and natural soil NPs from Luvisol, Phaeozem and Ferralsol under various solution chemistry were studied. The mechanisms were further elucidated by adopting the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The results indicated that the addition of negatively charged biochar NPs significantly altered the aggregation behaviours of positively charged hematite NPs through charge neutralisation. The aggregation of negatively charged illite particles was inhibited by the generating H-bonding and steric repulsion. Biochar NPs significantly increased the colloidal stability of Luvisol (from 343.82 to 382.96 mM) and Ferralsol NPs (from 28.39 to 215.35 mM) by increasing the net DLVO repulsive forces. Nevertheless, the stability of Phaeozem NPs, containing higher organic matter, decreased with increasing biochar NP concentrations due to electrostatic shielding. In conclusion, for complicated natural soil systems with significant differences between organic and inorganic components, the application of biochar NPs has a profound impact on colloidal particle interactions, particularly affecting positively-charged mineral colloids and soils with low soil organic matter content.